Question about isotopes?

Okay, I know that isotopes are atoms with the same number of protons and different numbers of neutrons, but in my physics book, when it's explaining how new elements are created from fusion, it says, "In the sun, different isotopes of hydrogen fuse, producing nuclei of helium." When they refer to isotopes that aren't mixed in with other atoms, is it just referring to an atom? I don't understand how you can determine whether an atom is an isotope without having other atoms to compare it to.